1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT) apparatus, and, more particularly, to a color CRT apparatus equipped with an electron gun which is designed to improve the withstand voltage characteristic and the focusing characteristic of the CRT.
2. Description of the Related Art
FIG. 1 shows the cross section of a typical color CRT apparatus. As shown in FIG. 1, a color CRT apparatus 1 is equipped with a vacuum envelope having a panel 3 with a phosphor screen 2, a funnel 4 extending from this panel 3, and a neck 5 coupled via this funnel 4 with the panel 3. An electron gun 6 is disposed inside the neck 5 of the vacuum envelope. A deflection yoke 7 is attached to the outer surface of the vacuum envelope which extends from the neck 5 to the funnel 4. A shadow mask 9 having a number of apertures 8 is disposed facing the inner wall of the phosphor screen 2 with a predetermined interval therebetween. An internal conductive film 10 is evenly coated on the inner wall of the vacuum envelope between the funnel 4 to a part of the neck 5. An outer conductive film 11 is coated on the outer surface of the funnel 4, with an anode terminal (not shown) provided at a part of the funnel 4.
A red fluorescent material, a green fluorescent material, and a blue fluorescent material are coated at many positions in a stripe or dot form, and three electron beams BR, BG, and BB launched from the electron gun 6 are properly selected by the shadow mask 9 to hit the respective fluorescent materials, causing the fluorescent materials to luminesce. The electron gun 6 has an electron-beam generating section GE, which generates three parallel in-line electron beams, and at the same time, controls and accelerates those electron beams, and a main electronic lens section ML, which converges and focuses those three electron beams. The three electron beams are deflected by the deflection yoke to scan the entire phosphor screen, showing an image on the phosphor screen.
The three electron beams may be converged, for example, by a technique disclosed in the specification of U.S. Pat. No. 2,957,106, wherein slightly inclined, unparallel electron beams, which are launched from the cathodes, are focused due to the inclination. Another technique of focusing the electron beams is disclosed in the specification of U.S. Pat. No. 3,772,554. According to this technique, the openings of some of the three electron-beam through holes formed on both sides of the electrodes in the electron gun are made slightly eccentric to the center axis of the electron gun to thereby converge the electron beams. Both of these techniques are widely employed.
The deflection yoke basically has a horizontal deflection coil for generating a horizontal-deflection magnetic field to deflect an electron beam in the horizontal direction, and a vertical deflection coil for generating a vertical-deflection magnetic field to deflect an electron beam in the vertical direction. When electron beams are deflected, the convergence of the spots of the three electron beams on the phosphor screen of an actual color CRT is shifted so that some measure is taken to prevent the misconvergence. This is called a convergence free (self-convergence type) system, which generates a horizontal-deflection magnetic field of a pin-cushion shape and a vertical-deflection magnetic field of a barrel shape to converge the three electron beams on the entire phosphor screen.
When the deflection magnetic fields are not formed unequal to each other as mentioned above, the resolution at the peripheral portion of the screen of the color CRT is reduced, and this tendency becomes more prominent as the deflection angle increases from 90 degrees to 110 degrees. This reduction in resolution at the peripheral portion of the screen occurs because the focusing is lessened in the horizontal direction by the deflection magnetic field shown in FIG. 2A while the focusing is intensified in the vertical direction by the deflection magnetic field shown in FIG. 2B. Accordingly, a beam spot 20 in the center portion of the screen would have a nearly circular shape while beam spots 21 at the peripheral portion of the screen would have a shape consisting of a high-luminance, horizontally-elongated elliptical core portion 23 and a low-luminance, vertically-elongated elliptical halo portion 24, as shown in FIG. 3.
To reduce the deformation of beam spots at the peripheral portion of the screen so as to improve the resolution, the techniques proposed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 60-7345 (corresponding U.S. Pat. No. 4,887,001), Jpn. Pat. Appln. KOKAI Publication No. 64-38947 (corresponding U.S. Pat. No. 4,897,575), and Jpn. Pat. Appln. KOKAI Publication No. 1-236554 (corresponding U.S. Pat. No. 5,034,652) are effectively. Particularly, the electron guns described in Jpn. Pat. Appln. KOKAI Publication No. 64-38947 and Jpn. Pat. Appln. KOKAI Publication No. 1-236554 can make the beam spot at the center of the screen smaller. Further, the color CRT described in Jpn. Pat. Appln. KOKAI Publication No. 64-38947 employs a so-called dynamic focusing technique of varying the intensity of the electronic lens of the electron gun in accordance with the amount of deflection to thereby make the deformation of the beam spot at the center of the screen very small. Using this technique, an image of high resolution over the entire screen can be obtained.
As described in the aforementioned KOKAI Publication No. 64-38947, asymmetric electronic lenses are formed in front of and at the back of a normal symmetric cylindrical electronic lens within the lens region. To form such asymmetric electronic lenses, an eaves-shaped electric-field compensating electrode 28 is placed inside a bathtub-shaped electrode 27 as shown in FIG. 4 according to the prior art.
In the color CRT described in Jpn. Pat. Appln. KOKAI Publication No. 64-38947, a resistor is provided inside the neck in the vicinity of the electron gun to supply the potential of a specific electrode of the electron gun, thereby accomplishing good dynamic focusing.
FIGS. 5A and 5B show the cross sections of the electron gun assembly of the prior art. In FIGS. 5A and 5B, an electron gun 6 has three cathodes KR, KG, and KB, each housing a heater (not shown), and arranged in a straight line, a first grid G1, a second grid G2, a third grid G3, a fourth grid G4, and a convergence cup CP. These components are arranged in the named order along the tube axis, and are securely supported by insulating supports MFG.
The grids G1 and G2 are thin plate-shaped electrodes of 0.2-mm in thickness. The grid G1 has three small electron-beam through holes AR1, AG1, and AB1 of about 0.7 mm in diameter bored through it at center distances of 6.6 mm. The grid G2, likewise, has three small electron-beam through holes AR2, AG2, and AB2 of about 0.7 mm in diameter bored through it at center distances of 6.6 mm.
The grid G3 comprises two bathtub-shaped electrodes 27-1 and 27-2 and an eaves-shaped electric-field compensating electrode 28-1 inserted therebetween. Three electron-beam through holes AR3-1, AG3-1, and AB3-1, 1.3 mm in diameter, are bored through the bathtub-shaped electrode 27-1 at the grid-G2 side. Three electron-beam through holes AR3-2, AG3-2, and AB3-2, 6.2 mm in diameter, are bored through the bathtub-shaped electrode 27-2 at the grid-G4 side. As shown in FIG. 6, the cylindrical shape of each of the bathtub-shaped electrodes 27-1 and 27-2 has an outside diameter L.sub.H of 21.3 mm in the long-axial direction and an outside diameter L.sub.V of 9.5 mm in the short-axial direction.
The eaves-shaped portion of the electric-field compensating electrode 28-1 is formed of a flat plate, about 1.2 mm thick, 3.0 mm long, and 19.0 mm wide, extending in parallel to the plane of the path of each electron beam to sandwich that plane.
The grid G4, like the grid G3, comprises two bathtub-shaped electrodes 27-3 and 27-4 and an eaves-shaped electric-field compensating electrode 28-2 inserted therebetween. The cylindrical convergence cup CP is closely attached to the screen side of the grid G4, with a spring BS attached to the outer surface of the distal end of the convergence cup. The spring BS is pressed against a conductive film 10 coated on the inner wall of the neck 5. The convergence cup CP is a cylinder with an open end, 0.32 mm in thickness and 22.0 mm in diameter, and has three electron-beam through holes formed through its bottom in association with the electron-beam through holes of the bathtub-shaped electrode 27-4 of the grid G4.
The components from the cathodes to the grid G4 are securely supported by the insulating supports MFG and are accommodated in the neck 5 having an inside diameter of 23.9 mm. Those electrodes are designed slightly smaller than the inside diameter of the neck 5 so as not to touch the glass neck. Since the electron-beam through holes are generally mode as large as possible to provide for the aperture of the electronic lens, the outside diameters of the bathtub-shaped electrodes are large, making the gap g' between the insulating supports and the side walls of the electrodes significantly narrower. This is illustrated in FIG. 6, which is a cross-sectional view taken along the line VI--VI in FIG. 5B.
With the above-described electrode structure, for example, a cutoff voltage of 200 V and a video signal are applied to the cathodes, a ground potential is applied to the grid G1, a voltage of 500 V to 1 KV is applied to the grid G2, a voltage of 5 KV to 10 KV is applied to the grid G3, and a high anode voltage of 25 KV is applied to the grid G4. The high anode voltage is applied to the grid G4 via the conductive film 10, the spring BS, and the convergence cup CP, while the other electrode potentials are applied via a stem pin STP at the lower end of the neck 5. The application of such potentials forms high-performance electronic lenses as described in Jpn. Pat. Appln. KOKAI Publication No. 1-236554.
However, the color CRT using this technique has a poor withstand voltage characteristic, which is critical to the color CRT. This shortcoming is due to the electric field discharged from an edge 29 of the eaves-shaped electric-field compensating electrode 28-1 inserted inside the bathtub-shaped electrode 27-2. Normally, a high voltage is applied between the electrodes to execute the voltage withstanding process to process projecting objects and eliminate dust particles during the manufacturing of a color CRT. As the edge 29 is located inside the bathtub-shaped electrode 27, the process can hardly be executed. The same is of course true of the grid-G4 side, and the compensating electrode 28-2 also causes the problem of a poor withstand voltage characteristic.
Further, in a color CRT having a resistor disposed in the proximity of the electron gun in the neck, the resistor interferes with the voltage withstanding process on the electrodes in the vicinity of the resistor, including that electrode to which a potential is applied by the resistor. This is because the electric discharge can be suppressed by the resistor, even when a high voltage is applied during the process. Accordingly, the projecting objects and dust particles remain, particularly between the electrodes and the insulating supports of the electrodes, so that minute discharge occurs in the vicinity of the electrodes during the normal function of the color CRT, thus causing adverse effects, such as altering the focusing of electron beams. When the electrode structure having the eaves-shaped electric-field compensating electrode 28 inserted inside the bathtub-shaped electrode 27 is used in such a color CRT, the withstand voltage characteristic is further impaired.
To produce good image characteristics over the entire screen of a color CRT, as described above, it is effective to use an electron gun which is designed to form asymmetric electronic lenses in front of and at the back of a normal symmetric cylindrical electronic lens within the lens region or have a resistor provided in the vicinity of the electron gun to supply the proper electrode potential. According to the prior art, however, the withstand voltage characteristic, which is critical to the color CRT, is poor.